There are ways you can sync different iPods to the same iTunes library, and also have different music on each.

To do this, plug in an iPod (either one) and go to your iPod on the left side. In the iPod menu, hit music on the top and select "Selected playlists, artists and genres". In the lists that appear, choose which you want to sync to your iPod. When you are satisfied, hit "Apply" in the bottom right corner then sync your iPod. This way, you have your music and your wife has her own. I hope this helps